1. Field of the Invention
The present invention generally relates to an scanning unit and specifically relates to an optical scanning unit.
2. Description of the Related Art
Image forming apparatuses that employ laser are being widely used for printing electronic images. Such an image forming apparatus typically includes a scanning unit and a photosensitive drum. The scanning unit includes a polygon scanner (e.g., a polygon mirror) that deflects laser toward the photosensitive drum, which is rotating, thereby scanning the surface of the photosensitive drum with the laser. Due to such scanning, a latent image is formed on the surface of the photosensitive drum. In image forming apparatuses, image density must be increased to have better image quality, and images must be output speedily to have better operability.
One approach to increase the image density and output speed is to rotate the polygon scanner speedily. However, if the polygon scanner is rotated a high speed, noise and power consumption increases and durability of the polygon scanner drops.
Another approach is to produce multiple beams from a light flux emitted from the light source. As disclosed in Japanese Patent Application Laid-open No. 2005-250319 and Japanese Patent Application Laid-open No. 2004-287292 following three schemes are available for realizing the multiple beams:
(1) combining a plurality of facet emitting lasers,
(2) using a one-dimensional array of the facet emitting lasers, and
(3) a scheme of using a two-dimensional array of vertical cavity surface emitting lasers (VCSELs).
In the scheme (1), a low price can be realized because general-purpose lasers can be used, but using a plurality of beams to stably maintain a relative positional relationship between the lasers and a coupling lens is difficult, and an interval between a plurality of scan lines formed on a scanning target surface (hereinafter, “scan line interval”) may possibly become uneven. Further, in the scheme (1), the number of light sources practically has a limit, and the density growth and the increase in speed also have limits. In the scheme (2), an even scan line interval can be formed, but this scheme has a disadvantage that a power consumption of an element is increased. Furthermore, when the number of light sources is extremely increased, a displacement amount of each beam from an optical axis of an optical system is increased, and a so-called beam quality may be degraded.
On the other hand, in the scheme (3), a power consumption is one digit smaller than that of the facet emitting laser, and more light sources can be two-dimensionally integrated.
Japanese Patent Application Laid-Open No. 2005-250319 discloses a light emitting device that includes a plurality of light sources in which a plurality of independent and modulatable light-emitting members are arranged in a two-dimensional array, and a coupling lens that couples the divergent light beams emitted from the light-emitting members. The light emitting device is mounted on an scanning unit that in turn is assembled in an image forming apparatus.
Japanese Patent Application Laid-Open No. 2004-287292 discloses an scanning unit that includes an array of surface-emitting lasers, and an image forming apparatus in which the scanning unit is assembled.
In the conventional scanning units, a beam pitch of the light beams in a sub-scanning direction over the target surface (hereinafter, “sub-scanning beam pitch”) deviates from a predetermined beam pitch. As the technology of high-density images forming improves day-by-day, it is necessary to prevent the image quality from deteriorating due to the deviation of the sub-scanning beam pitch.